1. Field of the Invention
The present invention generally relates to a tilt sensor, an optical pickup apparatus, and an optical disk apparatus. The present invention relates more particularly to a tilt sensor that detects the tilt of an object to a reference plane. The present invention further relates to a tilt measurement apparatus and an optical pickup that includes the tilt sensor, and to an optical disk apparatus that includes the optical pickup.
2. Description of the Related Art
Optical disk drives such as CD (compact disk) drives and DVD (digital versatile disk) drives are widely used as large capacity storage apparatuses for computers. An optical disk drive includes an optical pickup. The optical pickup records data on an optical disk by forming fine spots along spiral or concentric tracks preformed on the optical disk with a laser beam. The optical pickup reproduces the data by applying a laser beam to the optical disk and detecting the reflective light from the optical disk.
An optical pickup generally includes a light source, a photo detector, and an optical system. The optical system includes an object lens. The optical system leads a light beam emitted by the light source and forms a light spot on the recording surface of the optical disk. The optical system also leads the reflective light reflected by the recording surface of the optical disk to the photo detector. A signal detected by the photo detector includes not only the data but also information (servo information) required for controlling the optical pickup. The optical pickup is controlled in accordance with the servo information so that the optical pickup forms the light spot of a predetermined shape on a predetermined position on the optical disk.
The optical axis of the object lens is desired to be perpendicular to the recording surface of the optical disk so as to form the light spot of a predetermined size at a predetermined position on the recording surface and to detect the reproduced data and the servo information at high accuracy. However, the optical axis of the object lens may be slanted to the recording surface of the optical disk due to bend and eccentricity of the recording medium. If the optical axis of the object lens is too much slanted to the recording medium, the light spot may be degraded, and the signal including the reproduced data and the servo information may be degraded.
There is a laser auto collimator for detecting the slant of an object using a light beam. As shown in FIG. 23, the laser auto collimator generally includes a light source LD, a polarization beam splitter PBS, a λ/4 plate PX, an object lens L, and a position sensor PSD. The light source LD is a laser diode, for example. The light emitting point of the light source LD is positioned at the focal point of the object lens L. The polarization beam splitter PBS is positioned on the optical path of a light beam emitted by the light source LD, and is distant from the light source LD by a distance “b”. The polarization beam splitter PBS splits a reflective light from an object M traveling through the object lens L. The λ/4 plate PX is disposed between the polarization beam splitter PBS and the object lens L. The position sensor PSD is positioned on the optical path of the light beam split by the polarization beam splitter PBS, and is distant from the polarization beam splitter PBS by a distance “b”.
The light beam emitted by the light source travels to the object M through the polarization beam splitter PBS, the λ/4 plate PX, and the object lens L. If the object M is slanted to the plane perpendicular to the light axis of the object lens L by an angle θ, a reflective light from the object M goes into the object lens L with a deviation of an angle of 2θ from the light beam traveling to the object M. The reflective light further travels to the position sensor PSD via the λ/4 plate PX and the polarization beam splitter PBS. The angle θ is obtainable based on the following formula:θ=d/2f  (1)where d is the position of the reflective light detected by the position sensor PSD, and f(=a+b) is the focal distance of the object lens L.
However, a laser auto collimator requires an expensive position sensor and an object lens having a long focal distance. As a result, the using of the laser auto collimator holds back the cost reduction and down sizing of optical disk apparatuses.
Various tilt sensors have been proposed that do not hold back the cost reduction and down sizing of optical disk apparatuses.
Japanese Laid-Open Patent Application No. 57-179954 (document 1) discloses one of such tilt sensors.
The document 1 discloses an optical disk apparatus (referred to as an optical disk player therein) in which a photo detector split into two portions receives a reflective light from a recording medium, and each portion of the photo detector outputs optoelectronic signal. The tilt of the recording medium is determined based on a differential signal of the two optoelectronic signals.
It is actually difficult to position the photo detector at a designed position at high precision. As a result, the photo detector may fail to determine the tilt accurately due to offset imposed in the differential signal by the error in positioning. Change over time and change in temperature may also cause offset in the differential signal.
By the way, the amount of information to be stored in an optical disk is increasing year by year. The use of short wavelength laser is studied as one of techniques for increasing the recording density of an optical disk.
However, if the wavelength of laser beam is shortened without changing the shape of optical disk, the effect of wave front aberration (especially coma aberration) caused by a deviation between the directions of an object lens axis and the directions perpendicular to the recording surface (media tilt) increases. As a result, the shape of light spot may be degraded, and a signal output by the photo detector containing reproduced information and servo information may be degraded.
In the case of 660 nm wavelength laser, only media tilt in the directions perpendicular to a tangent of a track (radial tilt) is taken into consideration. However, if the wavelength of laser beam is further shortened, media tilt in the directions of the tangent of a track (tangential tilt) needs to be taken into consideration.
To solve these problems, various sensors for independently detecting tilts of an object in two directions are proposed in the following documents: Japanese Patent Laid-Open Applications No. 5-006561, No. 5-006562, and No. 8-320217.
Tilt sensors (tilt detection apparatuses) disclosed in the above documents, however, fail to solve the following problem, that is, since the reflective light from the object is received by a photo detector including multiple photo detecting units, and the tilt of the object is determined based on the position of a light spot, if the photo detector is disposed at a deviated position, the output signal from the photo detector may contain offset, and as a result, the tilt of the object can not be determined with high precision.